Color gamut is an important feature of color printing and imaging systems. It is a measure of the range of colors that can be produced using a given combination of colorants. It is desirable for the color gamut to be as large as possible. The color gamut of the imaging system is controlled primarily by the absorption characteristics of the set of colorants used to produce the image. Imaging systems typically employ three or more colorants, typically including cyan, magenta, and yellow in the conventional subtractive imaging system. It is also common for such systems to include an achromic colorant such as black.
The ability to produce an image containing any particular color is limited by the color gamut of the system and materials used to produce the image. Thus, the range of colors available for image reproduction is limited by the color gamut that the system and materials can produce.
Color gamut is often thought to be maximized by the use of so-called "block dyes". In The Reproduction of Colour 4th ed., R. W. G. Hunt, Pp 135-144, it has been suggested that the optimum gamut could be obtained with a subtractive three-color system using three theoretical block dyes where the blocks are separated at approximately 490 nm and 580 nm. This proposal is interesting but cannot be implemented for various reasons. In particular, there are no real colorants corresponding to the proposed block dyes.
Variations in the block dye concept are advanced by Clarkson, M., E., and Vickerstaff, T., in "Brightness and Hue of Present-Day Dyes in Relation to Colour Photography," Photo. J. 88b, 26 (1948). Three example shapes are given by Clarkson and Vickerstaff: Block, Trapezoidal, and Triangular. The authors conclude, contrary to the teachings of Hunt, that a trapezoidal absorption spectra may be preferred to a vertical sided block dye. Again, dyes having these trapezoidal spectra shapes are theoretical and are not available in practice.
Finally, both commercially available dyes and theoretical dyes were investigated in "The Color Gamut Obtainable by the Combination of Subtractive Color Dyes. Optimum Absorption Bands as Defined by Nonlinear Optimization Technique," J. Imaging Science, 30, 9-12. The author, N. Ohta, deals with the subject of real colorants and notes that the existing curve for a typical cyan dye, as shown in the publication, is the optimum absorption curve for cyan dyes from a gamut standpoint.
In spite of the foregoing teachings relative to color gamut, the coupler sets which have been employed in silver halide color imaging have not provided the range of gamut desired for modern imaging. It is therefore a problem to be solved to provide a coupler set which provides an increase in color gamut compared to the coupler sets heretofore used for silver halide imaging.